1. Field of the Invention
This invention relates generally to riser tensioner systems for use on offshore platforms and, more particularly, to a riser tensioner system that utilizes a compression spring to absorb oscillatory, vertical movement of the platform while supporting the riser.
2. Description of the Related Art
Increased oil consumption has led to exploration and drilling in difficult geographic locations that were previously considered to be economically unfeasible. As is to be expected, drilling under these difficult conditions leads to problems that are not present under more ideal conditions. For example, an increasing number of exploratory wells are being drilled in deep water, offshore locations in an attempt to locate more oil and gas reservoirs. These exploratory wells are generally drilled from floating platforms, leading to a set of problems peculiar to that environment.
As in any drilling operation, offshore drilling requires that drilling fluid must be circulated through the drill bit to cool the bit and to carry away the cuttings. This drilling fluid is normally delivered to the drill bit through the drill string and returned to the floating vessel through an annulus formed between the drill string and a large diameter pipe, commonly known as a riser. The riser typically extends between a subsea wellhead assembly and the floating vessel and is sealed against water intrusion.
The lower end of this riser is connected to the wellhead assembly adjacent the ocean floor, and the upper end usually extends through a centrally located opening in the hull of the floating vessel. The drill string extends longitudinally through the riser and into earth formations lying below the body of water, and drilling fluid circulates downwardly through the drill string, out through the drill bit, and then upwardly through the annular space between the drill string and the riser, returning to the vessel.
As these drilling operations progress into deeper waters, the length of the riser and, consequently, its unsupported weight also increases. Riser structural failure may result if compressive stresses in the elements of the riser exceed the metallurgical limitations of the riser material. Riser tensioner systems are typically used to avoid of this type of riser failure.
Riser tensioner systems are installed onboard the platform, and apply an upward force to the upper end of the riser, usually by means of cable, sheave, and pneumatic cylinder mechanisms connected between the vessel and the upper end of the riser.
In addition, buoyancy or ballasting elements may also be attached to the submerged portion of the riser. These usually are comprised of syntactic foam elements or individual ballast or buoyancy tanks formed on the outer surface of the riser sections. The ballast or buoyancy tanks are capable of being selectively inflated with air or ballasted with water by using the floating vessel's air compression equipment. These buoyancy devices create upwardly directed forces in the riser, and, thereby, compensate for the compressive stresses created by the riser's weight.
Both types of these mechanisms suffer from inherent disadvantages. Hydraulic and pneumatic tensioning systems are large, heavy, and require extensive support equipment, such as, air compressors, hydraulic fluid, reservoirs, piping, valves, pumps, accumulators, electrical power, and control systems. The complexity of these systems necessitate extensive and frequent maintenance with their attendant high cost.
The present invention is directed to overcoming or minimizing one or more of the problems set forth above.